When an electrical instrument is powered-up, if an operating switch is in an open state an engine of the electrical instrument will be inoperable and, if the operating switch is in a closed state, the engine will operate as soon as the electrical instrument is powered-up, which thus induces safety concerns.
According to new safety criterion, when the operating switch is in the closed state, the engine must not start operating when the electrical instrument is initially powered-up. To this end, many systems for controlling engine operation are composed of a microcontroller and accessory circuits. Although the microcontroller system does provide some protection, the microcontroller system is easy interfered with by the environment and can therefore suffer instability on power-up of the electrical instrument and cause safety issues. In this regard, the current through the power components in the control circuit and the voltage applied thereto are typically very high and the power components can therefore be easy damaged by such current and voltage. Similarly, many products using AC power use Silicon Controlled Rectifiers (SCRs) in serial in a main loop to control the circuit on/off, adjust speed, adjust voltage, etc., and electromagnetic interference can damage some elements in the main loop. When the elements in the control circuit are damaged, the engine may get out of control which exposes the user to further dangers. The new safety criterion therefore requires that if any element in the control circuit is damaged, the engine must not operate.